IC having comparator inputs connected to core circuitry and output pad

ABSTRACT

As IC circuit density increases, the testing of ICs becomes more complex and costly for the IC manufacturers. A test system and method achieves cost reduction in testing by permitting simultaneous testing of a plurality of semiconductor die. The testing can occur while the die are on a wafer or after the die have been packaged.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under priority under 35 USC 119(e)(1)of provisional application Ser. No. 60/215,247, filed Jun. 30, 2000.

FIELD OF THE INVENTION

The present invention relates generally to integrated circuits and, moreparticularly, to systems and methods for testing integrated circuits.

BACKGROUND OF THE INVENTION

As transistor geometry continues to shrink, more and more functionalcircuitry may be embedded within integrated circuits (ICs). This trendis beneficial for the electronics industry since it enables developmentof smaller, lower power electronic consumer products, such as cellphones and hand held computers. However, as IC circuit densityincreases, the testing of ICs becomes more complex and costly for the ICmanufacturers. Reducing the cost of manufacturing ICs is a primary goalfor every IC manufacturer By reducing IC manufacturing cost, an ICmanufacturer can advantageously cost-differentiate its IC products fromother IC manufacturers.

FIG. 1A illustrates a semiconductor wafer 101 comprising multiple die102 circuits. FIG. 1B illustrates one of the die circuits 101 on wafer101. The die contains core circuitry 103, which provides thefunctionality of the die, and pad locations 104 for providing contactsfor accessing the core circuitry.

FIG. 1C illustrates a conventional test arrangement for contacting andtesting a single die 102 of wafer 101. The test arrangement includes atester 105, a single die probe mechanism 109, and a die 102 to betested. Tester 105 comprises a controller 106, stimulus circuitry 108,and response circuitry 107. Controller 106 regulates the stimuluscircuitry 108 via interface 117 to output test stimulus signals to die102 via stimulus bus 111. Controller 106 regulates the responsecircuitry 107 via interface 118 to receive test response signals fromdie 102 via response bus 110. Probe mechanism 109 comprises the stimulusbus 111 and response bus 110 connection channels between tester 105 anddie 102. The probe mechanism contacts the input 115 and output 116 diepads via small probe needles 112. While only a pair of input and outputprobe needles 112 are shown in this simple illustration, it isunderstood that all die input and output pads will be similarlycontacted by the probe mechanism 109 using additional probe needles 112.The input pads 115 transfer stimulus signals to core 103 via inputbuffers 113, and the output pads 116 transfer test response signals fromcore 103 via output buffers 114. The testing of the die 102 in FIG. 1Coccurs through the process of inputting stimulus signals to the die andreceiving response signals from the die.

FIG. 2 illustrates in more detail the stimulus 108 and response 107circuitry of tester 105. Stimulus circuitry 108 typically comprises alarge stimulus data memory 201 for storing the stimulus data to beapplied to the die. Controller 106 controls the loading of the stimulusdata memory 201 from a source, such as a hard disk, prior to testing,and then controls the stimulus data memory to output the loaded stimulusdata to the die during test, via stimulus bus 111. Response circuitry107 typically comprises a large mask and expected data memory 203, acomparator 204, and a fail flag memory 202. The mask and expected datamemory 203 stores mask and expected data to be used by the comparator204 to determine if the response data from the die passes or fails.

During test, the comparator 204 inputs response signals from the die viaresponse bus 110, and mask (M) and expected (E) data signals from memory203 via mask and expected data buses 206 and 207. If not masked, by masksignal input from memory 203, a given response signal from the die iscompared against a corresponding expected data signal from memory 203.If masked, by mask signal input from memory 203, a given response signalfrom the die is not compared against an expected data signal from memory203. If a non-masked response signal matches the expected signal, thecompare test passes for that signal. However, if a non-masked responsesignal does not match the expected signal, the compare test fails forthat signal and the comparator outputs a corresponding fail signal onbus 205 to fail flag memory 202. At the end of test, the controller 106reads the fail flag memory to determine if the die test passed orfailed. Alternately, and preferably in a production test mode, thesingle die test may be halted immediately upon the controller receivinga compare fail indication from the fail flag memory 202, via theinterface 118 between controller 106 and response circuitry 107, toreduce wafer test time. At the end of the single die test, the probemechanism is relocated to make contact to another single die 102 ofwafer 101 and the single die test is repeated. The wafer test completesafter all die 102 of wafer 101 have each been contacted and tested asdescribed above.

FIG. 3 illustrates a conventional test arrangement for simultaneouslycontacting and testing multiple die 102 of wafer 101. The testarrangement includes tester 105, multiple die probe mechanism 301, and amultiple die 1-N 102 to be tested. The difference between the single andmultiple die test arrangements of FIGS. 2 and 3 is in the use of themultiple die probe mechanism 301. As seen in FIG. 3, the connectionbetween probe mechanism 301 and tester 105 is as previously described.However, the connection between probe mechanism 301 and die 1-N isdifferent. Each stimulus bus signal from the tester uniquely probescommon pad inputs on each die 1-N. For example, the stimulus 1 (S1)signal from the stimulus bus probes all common input pads 303 of all die1-N via connection 302. While not shown, stimulus 2-N (S2-N) signalsfrom the stimulus bus would each similarly probe all other common inputpads of all die 1-N. This allows the stimulus bus signals tosimultaneously input the same stimulus to all die 1-N during the test.

As seen in FIG. 3, the die response connection of probe mechanism 301 isdifferent from the above described die stimulus connection. Whereas eachcommon input pad 303 of die 1-N share a single stimulus signalconnection 302, each common output pad 304 requires use of a dedicatedresponse signal connection. For example, output pad 304 of die 1 uses aresponse signal connection 305, output pad 304 of die 2 uses a responsesignal connection 306, output pad 304 of die 3 uses a response signalconnection 307, and output pad 304 of die N uses a response signalconnection 308. All other output pads of die 1-N would similarly use adedicated response signal connection. All dedicated response signalconnections are channeled into the response bus to tester 105, as seenin FIG. 3. During test, the tester outputs stimulus to all die 1-N andreceives response outputs from all die 1-N. The test time of testingmultiple die in FIG. 3 is the same as testing single die in FIG. 2. Thetest operates in the masked/non-masked compare mode as described inFIGS. 1C and 2. When testing multiple die simultaneously, as opposed totesting a single die, a production test preferably runs to completioneven though an early compare may occur on one or more of the die beingtested. This is done because typically most of the die will pass theproduction test and aborting the multiple die production test on afailure indication would actually increase the test time, since the testwould need to be re-run later to complete the testing of the passingdie.

The limitation of the multiple die test arrangement in FIG. 3 lies inthe number of dedicated response inputs 305-308 the tester 105 canaccept on its response bus. For example, if the tester can accept 300response input signals and each die has 100 output pads, the multipledie test arrangement of FIG. 3 is limited to only being able to test 3die at a time. Testing 300 die on a wafer with this 3 die per testlimitation would require having to relocate the probe mechanism 301approximately 100 times to contact and test three die at a time. Thetime required to relocate the probe mechanism and repeat the die testsay 100 times consumes test time which increases the cost to manufacturethe die. It is possible to widen the response bus input of the tester tosay 600 inputs to allow testing 6 die at a time, but adding circuitry tothe tester to increase its response bus input width is expensive andthat expense would increase the cost of manufacturing die.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a semiconductor wafer.

FIG. 1B illustrates one of the die circuits on the wafer of FIG. 1A.

FIG. 1C illustrates a test arrangement for contacting and testing asingle die in accordance with the prior art.

FIG. 2 illustrates the stimulus and response circuitry of the testarrangement of FIG. 1C.

FIG. 3 illustrates a test arrangement for simultaneously contacting andtesting multiple die on a wafer in accordance with the prior art.

FIG. 4 illustrates a tester in accordance with the invention.

FIG. 5A illustrates an example of one of a plurality of mask andexpected data encoding circuits existing within the mask and expecteddata circuit of FIG. 4.

FIG. 5B is a truth table illustrating the operation of the encodingcircuit of FIG. 5A.

FIG. 6A illustrates a die having test circuits in accordance with oneembodiment of the invention.

FIG. 6B illustrates the test circuit of FIG. 6A in greater detail.

FIG. 7A illustrates the compare circuit of FIG. 6B in greater detail.

FIG. 7B is a truth table illustrating the operation of the comparecircuit of FIG. 7A.

FIG. 7C illustrates the pass/fail scan memory of FIG. 7A in greaterdetail.

FIG. 8A illustrates the trinary gate circuit of FIG. 7A in greaterdetail.

FIG. 8B is a truth table illustrating the operation of the trinary gatecircuit of FIG. 8A.

FIG. 8B illustrates the operation of the Unlock state machine of FIG.8A.

FIG. 9A illustrates a die having test circuits in accordance withanother embodiment of the invention.

FIG. 9B illustrates the test circuit of FIG. 9A in greater detail.

FIG. 10A illustrates the compare circuit of FIG. 9B in greater detail.

FIG. 10B is a truth table illustrating the operation of the comparecircuit of FIG. 10A.

FIG. 11A illustrates a die having test circuits in accordance withanother embodiment of the invention.

FIG. 11B illustrates the test circuit of FIG. 11A in greater detail.

FIG. 12A illustrates the compare circuit of FIG. 11B in greater detail.

FIG. 12B is a truth table illustrating the operation of the comparecircuit of FIG. 12A.

FIG. 13A illustrates the pass/fail scan memory of FIG. 12A in greaterdetail.

FIG. 13B illustrates a die having test circuits in accordance with theinvention coupled to a tester.

FIG. 14 illustrates a test system according to the present invention.

FIG. 15 illustrates an alternate view of the test system of FIG. 14.

FIG. 16 illustrates in detail the functional testing of the die in FIG.15.

FIG. 17 illustrates in detail the parallel scan testing of the die inFIG. 15.

FIG. 18A illustrates an IC having embedded cores and a test circuit inaccordance with the invention.

FIG. 18B illustrates the test circuit of FIG. 18A in greater detail.

FIG. 19 illustrates a wafer that has been processed to include built-inconnections for accessing common die input and common die output pads.

FIG. 19A illustrates a multiple wafer test system in accordance with theinvention.

FIG. 20 illustrates a test system according to the present invention forsimultaneously testing multiple packaged ICs.

DETAILED DESCRIPTION OF THE INVENTION

The present invention, as described in detail below, providesimprovements that overcome the limitations stated above usingconventional multiple die testing arrangements. Most notably, thepresent invention provides for significantly increasing the number ofdie that may be tested in parallel, without having to increase the widthof the tester's response bus.

The present invention improves multiple die testing by; (1) adaptingtesters to communicate with multiple die using a novel responsesignaling technique, and (2) adapting the die to be receptive to thetester's novel response signaling technique. Also, the present inventionimproves connectivity to multiple die on wafer by processing stimulusand response interconnects on the wafer to improve access to multipledie during test. In addition to its ability to improve the testing ofmultiple die on wafer, the present invention may also be usedadvantageously to improve the testing of multiple packaged ICs.

FIG. 4 illustrates a tester 401 according to the present invention.Tester 401 is similar to tester 105 in that it includes a controller 402similar to controller 106, stimulus circuitry 403 similar to stimuluscircuitry 108, and response circuitry 404. Controller 402 is connectedto stimulus 403 and response 404 circuitry via interfaces 414 and 415respectively. Response circuitry 404 includes the previously describedresponse circuitry section 107 and a new response circuitry section 405.Response circuitry 405 is the previously mentioned adaptation of thetester to support the new response signaling technique for testingmultiple die according the present invention.

Response circuitry 405 comprises an enable, mask, and expected datamemory 406, and mask (M) and expected (E) data encoding circuitry 407.Memory 406 outputs a mask (MSK) data bus 410, expected (EXP) data bus409, and an enable (ENA) bus 408 to encoding circuitry 407. Encodingcircuitry 407 outputs an encoded response bus 411. The encoded responsebus 411 of response circuitry 405 differs from the response bus 110 ofresponse circuitry 107 in that the encoded response bus 411 is an outputbus and the response bus 110 is an input bus. Both response busses 411and 110 reside on the input/output bus 413 to response circuitry 404.The role of response bus 110 of circuit 107 is reduced when using tester401 to test multiple die according to the present invention, as will bedescribed later. Memory 406 of response circuitry 405 is accessed by thecontroller 402 via interface 415 to load data into memory 406 prior totesting, and to operate the memory 406 to output mask, expected, andenable data to encoding circuitry 407 during test.

FIG. 5A illustrates an example of one of a plurality of mask andexpected data encoding circuits 501 existing within the mask andexpected data circuit 407. Circuit 501 receives a mask data signal 512from bus 410, an expected data signal 513 from bus 409, and an enablesignal 514 from bus 408, and outputs an encoded response signal 511 onbus 411. The mask 512 and expected 513 data signals are input to decoder501. Decoder 501 decodes the mask and expected signal inputs and outputscontrol signals 506-508 to the control input terminal of switches, forexample transistors, 503-505. One contact terminal of switch 503 isconnected to a ground reference voltage (Gnd) and the other terminalcontact is connected to the input 509 of voltage follower amplifier 510.One contact terminal of switch 504 is connected to a positive referencevoltage (Vdd) and the other terminal contact is connected to the input509 of voltage follower amplifier 510. One contact terminal of switch505 is connected to a mid-point reference voltage between Vdd and Gnd (½Vdd) and the other terminal contact is connected to the input 509 ofvoltage follower amplifier 510. Amplifier 510 receives the enable input514 to enable or disable its output.

The operation of encoding circuit 501 is best understood via the truthtable of FIG. 5B. When the enable input (ENA) 514 is low, the output ofamplifier 510 is disabled from driving the encoded response output 511.When ENA 514 is high, the encoded response 511 output modes of circuit501 are; (1) Gnd (Low) when mask data input 512 (MSK)=0 and expecteddata input 513 (EXP)=0, (2) Vdd (High) when MSK=0 and EXP=1, and (3) ½Vdd (Mask) when MSK=1. So, the encoding circuit 501 responds to MSK 512,EXP 513, and ENA 514 inputs to output appropriate Disable, Low, High, orMask conditions on the encoded response output 511. As mentioned,multiple circuits 501 will exist in the encoding circuit 407. Forexample, if the encoded response bus 411 contains 300 individual encodedresponse signals 511, 300 circuits 501 will exist in the encodingcircuit 407. Also, the width of the MSK bus 410, EXP bus 409, and ENAbus 408 will be 300 signals wide each, to supply the MSK 512, EXP 513,and ENA 514 inputs to the 300 circuits 501.

FIG. 6A illustrates how conventional 2-state output buffers of die 601are adapted according to the present invention. Die 601 is similar todie 102 in that it includes input pads 602, output pads 603, inputbuffer 604, and core circuitry 605. Die 601 differs from die 102 in thatit substitutes test circuits 606 for conventional 2-state output buffers114.

FIG. 6B illustrates test circuit 606 in more detail. Test circuit 606comprises a 3-state output buffer 607 coupled between the core output610 and output pad 603, and a compare circuit 608. Compare circuit 608inputs the core output signal 610, an input 614 from the output pad 603,a scan input signal 611, scan control signals 612, a test enable signal609, and a compare strobe signal 613. Compare circuit 608 outputs a scanoutput signal 615. The test enable signal 609 is also connected to thecontrol input of the 3-state output buffer 607. Test enable 609, scancontrol 612, and compare strobe 613 are inputs to the die 601 fromtester 401 via stimulus bus 111. Scan input 611 and scan output 615 ofmultiple compare circuits 608 are daisy-chained to allow the tester 401to serially input and output to multiple compare circuits 608 viastimulus bus 111 and response bus 112. It should be noted that in thisexample that output buffer 607 operates functionally as a 2-state outputbuffer. The reason buffer 607 is selected to be a 3-state type outputbuffer is for when test circuit 606 is placed into a test mode by thetest enable input 609.

During functional operation of the die, test enable 609 is low whichenables output buffer 607 and disables compare circuit 608. Infunctional mode, test circuit 606 operates as a conventional 2-stateoutput buffer from die 601. During test mode operation of the die, testenable 609 is high which disables output buffer 607 and enables comparecircuit 608. In test mode, test circuit 606 stops operating as aconventional 2-state output buffer and starts operating in the test modeas defined by the present invention. During test mode, tester 401 inputsencoded response signals from the encoded response bus 411 to comparecircuit 608 via the output pad 603 and connection 614.

FIG. 7A illustrate the compare circuit 608 in more detail. Comparecircuit 608 comprises trinary gate 701, exclusive OR (XOR) gate 702, ANDgate 703, and pass/fail scan memory 704. Trinary gate 701 inputs anencoded response signal 511 from a circuit 501 via connection 614, andoutputs an expected (EXP) data signal 705 and a mask (MSK) data signal706. XOR gate 702 inputs the core output signal 610 and the EXP dataoutput signal 705, and outputs a compare signal 707. AND gate 708 inputsthe compare signal 707 and the MSK data signal 706, and outputs acompare out signal 708. Pass/fail scan memory 704 inputs the compare outsignal 708, compare strobe signal 613, scan input signal 611, scancontrol signals 612, and the test enable signal 609, and outputs thescan output signal 615. The test enable signal 609 is also input totrinary gate 701, XOR gate 702, and AND gate 703. When test enable islow (i.e. functional mode of die) it disables the operation of gates701-707 such that they are not active to consume power or produce signalnoise during functional operation of the die. Also while test enable 609is low, the pass/fail latch (described below) of pass/fail scan memory704 is initialized to the pass indication state.

FIG. 7C illustrates in more detail the pass/fail scan memory 704.Pass/fail scan memory 704 comprises pass/fail latch comprising a D-FF709 (or other type of single bit memory) and OR gate 713, and a scancell comprising multiplexer 710 and D-FF 711. The pass/fail latch (i.e.Or gate 713 and FF 709) receives the compare output 708, compare strobe613, and test enable 609. Test enable 609 is input to the FF 709 resetinput to initialize FF 709 to a pass indication condition. Comparestrobe 613 is input to the FF 709 clock input. Compare out 708 and the Qoutput 712 of FF 709 are input to OR gate 713, which inputs to the Dinput of FF 709. The scan cell (i.e. multiplexer 710 and FF 711)receives the Q output 712 from FF 709, the scan input signal 611, andscan control inputs 612, and outputs the scan output signal 615.Optionally, the scan cell may receive a boundary scan input 714 so thatthe scan cell may be used as the capture and shift stage of an IEEE1149.1 boundary scan cell in addition to its use as a pass/failindication scan cell by the present invention. The boundary scan input714 would be connected to core output signal 610 to allow the scan cellto capture the data output from the core then shift the captured datafrom the IC, as described in the IEEE 1149.1 standard. The scan cell isoperable in response to the scan control inputs 612 to capture thestored Q output signal 712 into FF 711 via multiplexer 710, then shiftdata from scan input 611 to scan output 615 via multiplexer 710. Thescan control inputs 612 may come from a tester as previously mentioned,or they may be selectively connected to a test port on the die, such asan IEEE 1149.1 test access port. When operating the scan cells 704 asIEEE 1149.1 capture shift and stage elements, the scan control 612 tothe scan cells will be coupled to the 1149.1 test access port to allowIEEE 1149.1 control of the scan cells during boundary scan testing.

The operation of compare circuit 608 is best understood via the truthtable of FIG. 7B. When the test enable 609 is low, compare circuit 608is disabled except for the scan cell (710, 711) which remains operableto capture and shift data. The reason the scan cell remains enabled isbecause the scan cell may be shared between being used as a pass/failindication scan cell by the present invention and also as an IEEE 1149.1boundary scan cell associated with the output pad 603 of die 601, asmentioned above. The sharing of the scan cell as both a pass/failindication scan cell and as an IEEE 1149.1 boundary scan celladvantageously reduces test circuit area in the die. When test enable609 is high, the compare circuit 608 is enabled to perform testingaccording to the present invention.

While test enable 609 is high, a Gnd (Low) encoded response input 614from tester 401 causes trinary gate 701 to output a high on MSK 706 anda low on EXP 705. This test condition compares for an expected low logiclevel on core output 610. If the core output 610 is low, the compareoutput 708 from gate 703 will input a low (pass condition) to pass/faillatch (713, 709). In response to the compare strobe 613 that accompanieseach encoded response input 614 from the tester 401, the low input oncompare output 708 will be clocked into FF 709 of the pass/fail latch tostore the passing compare test result. If the core output 610 is high,the compare output 708 will input a high (fail condition) to thepass/fail latch. Again, in response to the accompanying compare strobe613, the high input on compare output 708 will be clocked into FF 709 tostore the failing compare test result. If a high (a fail condition) isclocked into FF 709, FF 709 will latch up with a high (fail condition)on its Q output, via the connection 712 to OR gate 713, andremain-latched high through out the remainder of test. This latch up isrequired to prevent the high (fail condition) from being overwrittenduring subsequent compare strobe inputs 613 to FF 709. This compare lowoperation of the present invention realizes the compare low operationdescribed in regard to tester 105 of FIGS. 1C and 2.

While test enable 609 is high, a Vdd (High) encoded response input 614from tester 401 causes trinary gate 701 to output a high on MSK 706 anda high on EXP 705. This test condition compares for an expected highlogic level on core output 610. If the core output 610 is high, thecompare output 708 from gate 703 will input a low (pass condition) topass/fail latch (713, 709). In response to the accompanying comparestrobe 613 the low input on compare output 708 will be clocked into FF709 of the pass/fail latch to store the passing compare test result. Ifthe core output 610 is low, the compare output 708 will input a high(fail condition) to the pass/fail latch. Again, in response to theaccompanying compare strobe 613, the high input on compare output 708will be clocked into FF 709 to store the failing compare test result. Asmentioned above, if a high (a fail condition) is clocked into FF 709,the pass/fail latch will latch up through out the remainder of the testto prevent the high failing condition from being overwritten duringsubsequent compare strobe inputs 613 to FF 709. This compare highoperation of the present invention realizes the compare high operationdescribed in regard to tester 105 of FIGS. 1C and 2.

While test enable 609 is high, a ½ Vdd (Mask) encoded response input 614from tester 401 causes trinary gate 701 to output a low on MSK 706. Thelow on MSK 706 forces the compare out 708 output of AND gate 703 low,which forces a pass condition to be clocked into the pass/fail latch,independent of the logic level output 707 from XOR gate 702. The testerinputs a ½ Vdd (Mask) encoded response input to trinary gate 701whenever it is not desired to perform a compare operation against thelogic level on core output 610. This mask operation of the presentinvention realizes the mask operation described in regard to tester 105of FIGS. 1C and 2.

FIG. 8A illustrates an example trinary gate 701 circuit. Trinary gate701 comprises p-channel transistor 801, current source 802, currentsource 803, n-channel transistor 804, OR gate 805, inverter 806, andtransmission gate switches 807 and 808. Transistor 801 and currentsource 802 form a first path between Vdd and Gnd. Transistor 804 andcurrent source 803 form a second path between Vdd and Gnd. A first nodebetween transistor 801 and current source 802 is connected to aninverted input of OR gate 805. A second node between transistor 804 andcurrent source 803 is connected to the other input of OR gate 805 and toinverter 806. The output of OR gate 805 is the Mask (MSK) Data signal706. The output of inverter 806 is the Expected (EXP) Data signal 705.The test enable 609 signal is connected as a control input to switches807 and 808. When test enable 609 is low, switch 807 connects the gateinput of transistor 801 to Vdd and switch 808 connects the gate input oftransistor 804 to Gnd, turning both transistor off and setting the firstand second nodes low and high respectively. When test enable 609 ishigh, switches 807 and 808 connect the gate inputs of transistors 801and 804 to the encoded response signal 614, enabling the transistors torespond to the encoded response signal.

The operation of trinary gate 701 is best understood via the truth tableof FIG. 8B. When the test enable 609 is low, transistors 801 and 804 aredisabled from responding to the encoded response signal 614 and the MSK706 and EXP 705 outputs are forced high and low respectively. While testenable 609 is low, the trinary gate 701 is disabled to reduce powerconsumption and noise during functional mode of the die, as previouslymentioned. While test enable 609 is high, and when a Gnd (Low) signal isinput on the encoded response input 614, the first and second nodes arehigh, producing a high on MSK signal 706 and a low on EXP signal 705.While test enable 609 is high, and when a Vdd (High) signal is input onthe encoded response input 614, the first and second nodes are low,producing a high on MSK signal 706 and a high on EXP signal 705. Whiletest enable 609 is high, and when a ½ Vdd (Mask) signal is input on theencoded response input 614, the first node is high and the second nodeis low, producing a low on MSK signal 706 and a high on EXP signal 705.During a ½ Vdd (Mask) input, the logic level output on the EXP 705signal is indicated in the truth table as a don't care (X) since thecompare operation is masked by the low on MSK signal 706.

While not shown, the test enable signal 609 input to XOR gate 702 andAND gate 703 can be used to disable their input threshold transistorsand set their outputs to static DC low states similar to the way it isshown doing so in the trinary gate 701 of FIG. 8A. Again, this is doneto reduce power and noise of comparators 608 during functional operationof die 601.

FIG. 9A illustrates how conventional 3-state output buffers of die 601are adapted according to the present invention. Die 601 of FIG. 9A isthe same as die 601 of FIG. 6A with the exception that FIG. 9Aillustrates how test circuits 906 are substituted for conventional3-state output buffers between core 605 and 3-state output pads 903.Similar to die 601 of FIG. 6A, die 601 of FIG. 9A includes input pads602, input buffers 604, core circuitry 605, and 3-state output pads 903as opposed to 2-state output pads 603 in FIG. 6A. Die 601 of FIG. 9Adiffers from die 601 of FIG. 6A in that it illustrates the substitutionof test circuits 906 for conventional 3-state output buffers at outputpads 903, instead of the substitution of test circuits 606 forconventional 2-state output buffers at pads 603.

FIG. 9B illustrates test circuit 906 in more detail. Test circuit 906comprises a 3-state output buffer 907 coupled between the core output910 and output pad 903, an AND gate 901, and a compare circuit 908. ANDgate 901 receives an output control signal 911 from core 605 on oneinput and an inverted test enable signal 609 on the other input. The ANDgate 901 outputs a 3-state control signal 902 to the 3-state buffer 907.Compare circuit 908 inputs the core output signal 910, core outputcontrol signal 911, an input 914 from the output pad 903, a scan inputsignal 611, scan control signals 612, a test enable signal 609, and acompare strobe signal 613. Compare circuit 908 outputs a scan outputsignal 615. Scan input 611 and scan output 615 of multiple comparecircuits 908 and 608 are daisy-chained to allow the tester 401 toserially input and output to multiple compare circuits 908 and 608 viastimulus bus 111 and response bus 112. It should be noted that in thisexample that output buffer 907 operates functionally as a 3-state outputbuffer, as opposed to output buffer 607 of FIG. 6B which operatesfunctionally as a 2-state output buffer. As with buffer 607, the outputof buffer 907 is disabled when test circuit 906 is placed into a testmode by the test enable input 609, via AND gate 901.

During functional operation of the die, test enable 609 is low whichenables output control signal 911 from core 605 to pass through gate 901to functionally enable and disable output buffer 907. In this example,and during functional operation, a low input on output control 911 willdisable the output of output buffer 907, and a high input on outputcontrol 911 will enable the output of output buffer 907. When outputcontrol in low, Also a low on test enable 609 disables compare circuit908. In functional mode, test circuit 906 operates as a conventional3-state output buffer from die 601. During test mode operation of thedie, test enable 609 is high which disables output buffer 907, via gate901, and enables compare circuit 908. In test mode, test circuit 906stops operating as a conventional 3-state output buffer and startsoperating in the test mode as defined by the present invention. Duringtest mode, tester 401 inputs encoded response signals from the encodedresponse bus 411 to compare circuit 908 via the output pad 903 andconnection 914.

FIG. 10A illustrates the compare circuit 908 in more detail. Comparecircuit 908 comprises trinary gate 701, XOR gate 702, AND gate 1003, andpass/fail scan memory 704. Trinary gate 701 inputs an encoded responsesignal 511 from a circuit 501 via connection 914, and outputs anexpected (EXP) data signal 705 and a mask (MSK) data signal 706. XORgate 702 inputs the core output signal 910 and the EXP data outputsignal 705, and outputs a compare signal 707. AND gate 708 inputs thecompare signal 707, output control signal 911, and the MSK data signal706, and outputs a compare out signal 1008. Pass/fail scan memory 704inputs the compare out signal 1008, compare strobe signal 613, scaninput signal 611, scan control signals 612, and the test enable signal609, and outputs the scan output signal 615. The test enable signal 609is also input to trinary gate 701, XOR gate 702, and AND gate 1003 toreduce power consumption and noise during functional die operation, asdescribed previously in regard to comparator 608. The pass/fail scanmemory operates as previously described in regard to FIG. 7C.

The operation of compare circuit 908 is best understood via the truthtable of FIG. 10B. When the test enable 609 is low, compare circuit 908is disabled except for the scan cell (710, 711) of pass/fail scan memory704 to enable sharing of the scan cell as both a pass/fail indicationscan cell and as an IEEE 1149.1 boundary scan cell as mentioned inregard to FIG. 7C. When test enable 609 is high, the compare circuit 908is enabled to perform testing according to the present invention.

While test enable 609 and output control 911 is high, a Gnd (Low)encoded response input 914 from tester 401 causes trinary gate 701 tooutput a high on MSK 706 and a low on EXP 705. This test conditioncompares for an expected low logic level on core output 910. If the coreoutput 910 is low, the compare output 1008 from gate 1003 will input alow (pass condition) to pass/fail latch (713, 709). In response to theaccompanying compare strobe 613, the low input (pass condition) isstored into the pass/fail latch, as previously described in regard toFIG. 7C. If the core output 910 is high, the compare output 1008 willinput a high (fail condition) to the pass/fail latch. In response to theaccompanying compare strobe 613 the high (fail condition) is stored andlatched in pass/fail latch as previously described in regard to FIG. 7C.

While test enable 609 and output control 911 is high, a Vdd (High)encoded response input 914 from tester 401 causes trinary gate 701 tooutput a high on MSK 706 and a high on EXP 705. This test conditioncompares for an expected high logic level on core output 910. If thecore output 910 is high, the compare output 1008 from gate 1003 willinput a low (pass condition) to pass/fail latch (713, 709). In responseto the accompanying compare strobe 613, the low input (pass condition)is stored into the pass/fail latch, as previously described in regard toFIG. 7C. If the core output 910 is low, the compare output 1008 willinput a high (fail condition) to the pass/fail latch. In response to theaccompanying compare strobe 613 the high (fail condition) is stored andlatched in the pass/fail latch as previously described in regard to FIG.7C.

While test enable 609 and output control 911 is high, a ½ Vdd (Mask)encoded response input 914 from tester 401 causes trinary gate 701 tooutput a low on MSK 706. The low on MSK 706 forces the compare out 1008output of AND gate 1003 low, which forces a low (pass condition) to bestored into the pass/fail latch in response to the accompanying comparestrobe 613, independent of the logic level output 707 from XOR gate 702.The tester inputs a ½ Vdd (Mask) encoded response input to trinary gate701 whenever it is not desired to perform a compare operation againstthe logic level on core output 910, as previously described in regard toFIGS. 7A and 7B.

While test enable 609 is high and output control 911 is low, a low (passcondition) is forced on the compare output 1008 of AND gate 1003. Thisforces a low (pass condition) to be stored into the pass/fail latch inresponse to the accompanying compare strobe 613, independent of thelogic level output 707 from XOR gate 702. This forced pass condition isdifferent from the forced pass condition controlled by tester 401 usingthe ½ Vdd input, since the core's output control signal 911 regulatesthe masking of the compare operation. This new mode of compare maskingenables testing the core's output control signal 911. For example, if,during a time in the test when the output control signal 911 should below, an intentionally failing encoded response signal 914 can be inputto the trinary gate 701. If the control output signal 911 is functioningproperly, it will mask the intentional failure input and force thecompare output 1008 of gate 1003 low (pass condition). However, if theoutput control signal 911 fails to function properly, it will not maskthe intentional failure input and the compare output signal 1008 will beset high (fail condition). There is a possibility that a faulty coreoutput signal 910 may compare equal to the intentional failure inputsignal 914, which will mask the test for a faulty output control signal911. For example, a faulty output control signal 911 may remain high(first fault) to allow a faulty core output signal 910 to pass thecompare test (second fault) and input a low (pass condition) to thepass/fail latch. To test for this possibility, two tests are run. Afirst test using the intentional failure input, and a second test usingthe actual expected data input. If both tests pass, then both the outputcontrol signal 911 and core output signal 910 are functioning properly.

FIG. 11A illustrates how conventional input/output (I/O) buffers of die601 are adapted according to the present invention. Similar to die 601of FIG. 9A, die 601 of FIG. 11A includes input pads 602, input buffers604, core circuitry 605, and I/O pads 1103 as opposed to 2-state and3-state output pads 603 and 903 in FIGS. 6A and 9A. Die 601 of FIG. 11Adiffers from die 601 of FIGS. 6A and 9A in that it substitutes testcircuits 1106 for conventional I/O buffers at output pads 1103, insteadof the substitution of test circuits 606 and 906 for conventional2-state and 3-state output buffers at pads 603 and 903.

FIG. 11B illustrates test circuit 1106 in more detail. Test circuit 1106comprises a 3-state output buffer 907 coupled between core output 1110and I/O pad 1103, an input buffer 1115 coupled between I/O pad 1103 andcore input 1112, an AND gate 901, and a compare circuit 908. AND gate901 receives an I/O control signal 1111 from core 605 on one input andan inverted test enable signal 609 on the other input. The AND gate 901outputs a 3-state control signal 902 to the 3-state buffer 907. Comparecircuit 908 inputs the core output signal 1110, core I/O control signal1111, an input 1114 from I/O pad 1103, a scan input signal 611, scancontrol signals 612, a test enable signal 609, and a compare strobesignal 613. Compare circuit 908 outputs a scan output signal 615. Scaninput 611 and scan output 615 of multiple compare circuits 908 and 608are daisy-chained to allow the tester 401 to serially input and outputto multiple compare circuits 908 and 608 via stimulus bus 111 andresponse bus 112.

FIG. 12B shows the compare circuit 908 of FIG. 11B in more detail. Thestructure and operation of compare circuit 908 of FIG. 12A is the sameas compare circuit 908 of FIG. 10A. The only structural differencebetween the two compare circuits 908 is that the I/O control signal 1111of FIG. 12A has been substituted for the output control signal 911 ofFIG. 10A. As seen in truth table 12B, compare circuit 908 of FIG. 12Aperforms all the functions of compare circuit 908 of FIG. 10A. Inaddition to these functions, compare circuit 908 of FIG. 12A supportsthe input stimulus function described below.

During conventional testing, tester 105 of FIG. 1C inputs stimulus viastimulus bus 111 and outputs response via response bus 110 toconventional IC I/O pads. During testing according to the presentinvention, tester 401 of FIG. 4 inputs stimulus using either stimulusbus 414 or encoded response bus 411, and outputs encoded response viaencoded response bus 411 to IC I/O pads 1103. In either test case, theI/O control signal 1111 will select the input or output function bycontrolling the output condition of 3-state buffer 907. For example,when the I/O control signal 1111 of test circuit 1106 in FIG. 11B is setlow, the output of the 3-state buffer 907 is disabled to allow thetester 401 to input stimulus to core 605 from I/O pad 1103. The stimulusinput from the tester 401 is input using conventional logic low (Gnd)and high (Vdd) voltage levels, which as mentioned can come from eitherthe stimulus bus 414 or encoded response bus 411. As seen in FIG. 12A,the low on I/O control signal 1111 that selects the stimulus input modealso forces the output 1008 of AND gate 1003 low to input passconditions to pass/fail flag in pass/fail scan memory 704. This is doneto prevent a high (fail condition) from being unintentionally stored andlatched in the pass/fail flag, in response to accompanying comparestrobes 613, during times when the tester 401 is inputting stimulus.

As mentioned previously in regard to FIG. 3, production testing ofmultiple die preferably runs to completion without regard to one or moredie incurring failures during the test. However, during diagnostictesting of multiple die it is advantageous to be able to detect a firstfailure to allow determining the exact test pattern that caused thefailure. To provide for diagnostic testing using the present invention,the pass/fail scan memory 704 is modified as follows.

In FIG. 13A, the pass/fail scan memory 704 is shown to include anadditional transistor 1301. The transistor has one terminal connected toGnd and the other terminal connected to a fail output signal 1302, whichis externally output from the pass/fail scan memory 704. The gate inputof transistor 1301 is connected to the Q output signal 712 of FF 709.While the Q output 712 is low (pass condition), the transistor is offand the fail output signal 1302 is isolated from Gnd. When the Q outputis high (fail condition), the transistor is on and a conduction path isenabled between fail output signal 1302 and Gnd. As can be seen,transistor 1301 operates as an open drain, isolating the fail outputsignal 1302 from Gnd while Q is low (pass condition), and connecting thefail output signal 1302 to Gnd when Q is high (fail condition).

FIG. 13B illustrates and a die 1303 coupled to a tester 401. Die 1303includes mixtures of the previously described test circuits 608 and 908.The test circuits 608 and 908 each contain the pass/fail fail output1302 equipped scan memory 704 of FIG. 13A. The fail outputs 1302 of eachtest circuit 608 and 908 are externally available to be connected to abussed fail output signal 1304 within the die. The bussed fail outputsignal 1304 is also connected to a current source 1305, which serves asa pull element for the bussed fail output signal 1304. The bussed failoutput signal 1304 is externally output from the die as a fail output totester 401. While the pull up element 1304 is shown existing inside thedie, it could exist external of the die as well, i.e. the tester 401could provide the pull up element 1305.

Diagnostic testing of multiple die 1303 using the present invention issimilar to the previously described production test using the presentinvention. However, unlike production testing, diagnostic testing willbe halted upon the first compare failure to enable identification of thedie test pattern that failed, so that the nature of the failure may beanalyzed. During diagnostic testing, the test circuits 608, 908 of themultiple die perform the compare operations between the core outputs610, 910, 1110 and encoded response inputs 614, 914, 1114. As can beseen from FIG. 13A, when a first high (fail condition) is stored andlatched in FF 709, the gate of transistor 1301 is driven high by the Qoutput of FF 709. With the gate input high, the transistor 1301 is onand forms a conduction path between fail output 1302 and Gnd. As can beseen in FIG. 13B, when one or more transistors 1301 turn on in responseto a fail condition, the bussed fail output connection 1304 is pulledlow (Gnd). The tester responds to this low level transition on the failoutput to halt the diagnostic test and to scan out the pass/fail flagsof the daisy-chained test circuits 608/908. By inspecting the scannedout pass/fail flag bits, the tester can determine which one or more coreoutput signal(s) failed. Thus the present invention supports diagnostictesting of multiple die if the pass/fail scan memory 704 of FIG. 13A isused in place of the previously described pass/fail scan memory 704 ofFIG. 7C.

FIG. 14 illustrates a test system according to the present invention.The test system comprises a tester 401, a multiple die probe mechanism1401, and die 1-N to be tested. The probe mechanism 1401 is similar tothe probe mechanism 301 of FIG. 3 in that it has a stimulus channel 302for probing all common input pads 303 of die 1-N. Probe mechanism 1401differs from probe mechanism 301 in that it has an encoded responsechannel for probing all common output pads 1402 of die 1-N. During test,all die 1-N receive a common stimulus input on each common input padinput 303, and all die 1-N receive a common encoded response input oneach common output pad 1402. From inspection of the probe mechanism ofFIG. 14, it is seen that the test system of the present invention doesnot suffer from the previously mentioned tester response channellimitation mentioned in regard to the conventional test system of FIG.3. For example, if the tester 401 has 300 stimulus channels and 300response channels, and die 1-N have 300 or less input pads and 300 orless output pads, any number of die 1-N may be simultaneously testedusing the test system of the present invention. Thus, use of the testsystem of FIG. 14 reduces the test time of the die on wafer, andtherefore reduces the cost to manufacture the die.

FIG. 15 illustrates an alternate view of the test system of FIG. 14.Tester 401 is illustrated as the outer layer, probe mechanism 1401 isillustrated as being inside the tester 401 layer, and wafer 1501 withdie 1-N is illustrated as being inside the probe mechanism layer 1401.Each die 1-N are identical and each have inputs 1-M connected to inputpads 1502-1504 and 2-state outputs 1-N connected to output pads1505-1507. The stimulus bus 414 from the tester passes through the probemechanism to the die input pads 1502-1504. The encoded response bus 411and response bus 110 from the tester pass through the probe mechanism tothe die output pads 1505-1507. Common input pads 1502 of die 1-N areconnected together and to one stimulus channel from stimulus bus 414 viathe probe mechanism, common input pads 1503 are connected together andto another stimulus channel from stimulus bus 414 via the probemechanism, and inputs pads 1504 are connected together and to a furtherstimulus channel from stimulus bus 414 via the probe mechanism. Commonoutput pads 1505 of die 1-N are connected together and to one encodedresponse channel from encoded response bus 411 via the probe mechanism,common output pads 1506 are connected together and to another encodedresponse channel from encoded response bus 411 via the probed mechanism,and common output pads 1507 are connected together and to a furtherencoded response channel from encoded response bus 411 via the probemechanism.

The pass/fail scan input 611 from the tester passes through the probemechanism 1401 to the scan input of die 1, through the daisy-chainedscan path of die 1-N to be output on the pass/fail scan output 615 tothe tester via probe mechanism 1401. The scan input 611 uses one of thestimulus input channels of stimulus bus 414 and the scan output uses oneof the response output channels of response output bus 110. While thescan control signals 612, test enable signal 609, and compare strobesignal 613 are not explicitly shown in FIG. 15, they are also connectedto die 1-N inputs 1-M via stimulus channels from stimulus input bus 414.While test circuits 606 are shown existing on die 1-N 2-state outputpads 1505-1507, it should be clear that test circuits 906 would exist ondie 1-N 3-state output pads 1505-1507, and test circuits 1106 wouldexist on die 1-N I/O pads 1505-1507. If test circuits 1106 were used ondie I/O pads 1505-1507, then the encoded response bus 411 would be usedto input stimulus data to the I/O pads 1505-1507, via probe mechanism1401, as described in regard to FIGS. 12A and 12B. Thus is this example,the encoded response bus 411 serves the dual role of; (1) inputtingencoded response signals to I/O pads during compare/mask operations, and(2) inputting stimulus data to I/O pads during stimulus inputoperations.

FIG. 16 illustrates in detail the functional testing of die 1-N(1601-1603) of FIG. 15. Tester 401 inputs stimulus from stimulus bus 414to common die inputs 1502-1504 via the connections 1609-1611, to allowall die 1-N to receive the same stimulus at their common inputs duringtest. Connections 1609-1611 are provided by the probe mechanism 1401 ofFIGS. 14 and 15. Also, tester 401 inputs stimulus from stimulus bus 414to the scan input 611 of die 1 via the probe mechanism.

Tester 401 inputs encoded response inputs from encoded response bus 411to common die outputs and I/Os 1505-1507 via the connections 1606-1608,to allow all die 1-N to receive the same encoded response inputs attheir common outputs and I/Os during test. Connections 1606-1608 areprovided by the probe mechanism 1401 of FIGS. 14 and 15. Tester 401inputs a combined fail output signal from die 1-N to response bus 110via the fail output connection 1605 provided by the probe mechanism.Also, tester 401 inputs the scan output signal 615 from die N to theresponse bus 110. Connection 1604 illustrates the daisy-chaining of thepass/fail scan output from die 1 to the pass/fail scan input of die 2,and so on to die N. Connection 1604 is provided by the probe mechanism.As seen in FIG. 16, encoded response input 1505 is coupled to 1-N2-state test circuits 606, encoded response input 1506 is coupled to 1-N3-state test circuits 906, and encoded response input 1507 is coupled to1-N I/O test circuits 1106.

During test, tester 401 places the die 1-N in the test mode of thepresent invention and inputs stimulus patterns to die 1-N inputs viaconnections 1502-1504 and inputs encoded response patterns to die 1-Ntest circuits 606, 906, and 1106 via connections 1505-1507. In responseto the functional patterns to the inputs and I/Os, die 1-N operates tooutput data to test circuits 606, output data and control to testcircuits 906, and input and output data and control to test circuits1106. During the test, tester 401 inputs the compare strobe to testcircuits 606, 906, and 1106 as previously described to store the compareresults between the functional output data and the encoded responseinput data from the tester. If the test is a production test, the failoutput from connection 1605 is ignored during the test for the reasonspreviously mentioned in regard to FIG. 3. If the test is a diagnostictest, the fail output from connection 1605 is monitored by the tester401 for the reasons previously mentioned in regard to FIGS. 13A and 13B.At the end of a functional production test or at the stopping of afunctional diagnostic test, tester 401 scans out the pass/fail flags inthe pass/fail scan memories of die 1-N via the scan input 611 and scanoutput 615 connections. From the pass/fail scan operation, the testercan determine if a failure occurred in die 1-N and if so identify thelocation of the failure.

FIG. 17 illustrates in detail the parallel scan testing of die 1-N(1701-1703) of FIG. 15. The difference between die 1-N of FIG. 16 anddie 1-N of FIG. 17 is that die 1-N of FIG. 17 have been designed to betested using a parallel scan design for test approach, whereas die 1-Nwere not and had to be tested functionally. When die 1-N are placed inthe parallel scan test configuration, the data inputs of scan paths 1-Nare connected to die inputs 1502-1504 and the data outputs of scan paths1-N are connected to the inputs 910 of test circuits 606. Tester 401inputs scan stimulus from bus 414 to die 1-N scan paths 1-N via thecommon die input connections 1502-1504 and 1609-1611, to allow all die1-N to receive the same scan stimulus during test. Also, tester 401inputs stimulus from bus 414 to the scan input 611 of die 1 via theprobe mechanism.

Tester 401 inputs encoded scan response from bus 411 to common dieoutput connections 1505-1507 and 1606-1608, to allow all die 1-N tocompare against the same response during test. Tester 401 inputs acombined fail output signal from die 1-N to response bus 110 via thefail output connection 1605. Also, tester 401 inputs the scan outputsignal 615 from die N to the response bus 110. Connection 1604illustrates the daisy-chaining of the pass/fail scan output from die 1to the pass/fail scan input of die 2, and so on to die N. As seen inFIG. 17, encoded scan response inputs 1505-1507 are coupled to 1-N2-state test circuits 606.

During test, tester 401 places the die 1-N in the test mode of thepresent invention and inputs stimulus patterns to scan paths 1-N of die1-N via inputs 1502-1504 and inputs encoded response patterns to testcircuits 606 of die 1-N via outputs 1505-1507. The scan paths operate,in response to conventional scan path control input from tester 401, toshift in the stimulus patterns from inputs 1502-1504, capture responsepatterns, and shift out the captured response patterns to test circuits606. During the test, tester 401 inputs the compare strobe to testcircuits 606 as previously described to store the compare resultsbetween the captured response data from scan paths 1-N and the encodedresponse input data from tester 401. If the test is a production test,the fail output from connection 1605 is ignored during the test for thereasons previously mentioned in regard to FIG. 3. If the test is adiagnostic test, the fail output from connection 1605 is monitored bythe tester 401 for the reasons previously mentioned in regard to FIGS.13A and 13B. At the end of a parallel scan production test or at thestopping of a parallel scan diagnostic test, tester 401 scans out thepass/fail flags in the pass/fail scan memories of die 1-N via the scaninput 611 and scan output 615 connections. From the pass/fail scanoperation, the tester can determine if a failure occurred in die 1-N andif so identify the location of the failure.

It is becoming increasingly popular to design systems on ICs usingpre-existing intellectual property core sub-circuits. Core sub-circuitsprovide embeddable functions such as DSP, CPU, and RAM. FIG. 18Aillustrates an IC comprising embedded cores 1-3. The cores are connectedtogether via functional connections 1814 and 1815 to form a system onthe IC. The following describes how such systems on ICs can be testedusing the present invention.

To test the embedded cores 1-3 of IC 1802, test connections 1810 andconnection circuits 1808 and 1809 are added to allow input pads 1803 tobe selectively connected to the inputs of cores 1-3. Also testconnections 1811, 1812 and 1818 are added to allow the outputs of cores1-3 to be connected to test circuits 1813, which are coupled to outputpads 1802. As seen in FIG. 18B, test circuit 1813 is similar to testcircuit 606 with the exception that it contains a multiplexer 1816 forreceiving core 1-3 outputs 1811, 1812, and 1818 and a core select input1817 for selecting which of the core 1-3 outputs 1811, 1812, or 1818will be selected for input to buffer 607 and compare circuit 608.

During the testing of core 1, the IC of FIG. 18A is configured such thatthe inputs to core 1 are coupled to input pads 1803 and the outputs fromcore 1 are coupled to test circuits 1813 via connections 1811. Also testcircuit 1813 is configured by the core select signals 1816 to connectthe core 1 outputs to compare circuits 608. After the IC has beenconfigured, core 1 is rendered testable using the present invention byinputting stimulus to core 1 via pads 1803 and inputting encodedresponse to test circuit 1813 via pads 1802 to compare against theoutputs from core 1. The testing of core 1 is as previously described inFIGS. 6A and 6B.

During the testing of core 2, the IC of FIG. 18A is configured such thatthe inputs to core 2 are coupled to input pads 1803, via connection 1810and connection circuit 1808, and the outputs from core 2 are coupled totest circuits 1813 via connections 1812. Also test circuit 1813 isconfigured by the core select signals 1816 to connect the core 2 outputsto compare circuits 608. After the IC has been configured, core 2 isrendered testable using the present invention by inputting stimulus tocore 2 via pads 1803 and inputting encoded response to test circuit 1813via pads 1802 to compare against the outputs from core 2. The testing ofcore 2 is as previously described in FIGS. 6A and 6B.

During the testing of core 3, the IC of FIG. 18A is configured such thatthe inputs to core 3 are coupled to input pads 1803, via connection 1810and connection circuit 1809, and the outputs from core 3 are coupled totest circuits 1813 via connections 1818. Also test circuit 1813 isconfigured by the core select signals 1816 to connect the core 3 outputsto compare circuits 608. After the IC has been configured, core 3 isrendered testable using the present invention by inputting stimulus tocore 3 via pads 1803 and inputting encoded response to test circuit 1813via pads 1802 to compare against the outputs from core 3. The testing ofcore 3 is as previously described in FIGS. 6A and 6B.

The individual core 1-3 tests described above could be performedsimultaneously on multiple ICs of FIG. 18A as described in regard FIG.15, which would lower the cost to manufacture the ICs of FIG. 18A.

FIG. 19 illustrates a wafer 1901 which has been processed to includebuilt-in connections for accessing common die input (S1) and common dieoutput (R1) pads. The wafer comprises; (1) die 1-N each with input (S1)pads and output (R1) pads, (2) stimulus input grid lines 1904 connectedto common die input pads, (3) encoded response input grid lines 1905connected to common die output pads, (4) pad fuses 1906 connected inseries between grid lines 1905 and pad connection lines 1907, 1908,1909, and 1910, (5) tester probe contacts 1903 for connecting tostimulus grid lines 1904, and (6) tester probe contacts 1902 forconnecting to encoded response grid lines 1905.

Tester 401 probes grid line contacts 1903, 1902 using a simplifiedexternal probe mechanism to input stimulus to the commonly connected dieinput pads and to input encoded response to the commonly connected dieoutput pads. Testing occurs on the die as previously described. Thedifference between the test systems of FIG. 19 and FIG. 14 is that inFIG. 19 most of the common pad connections are provided on the wafer1901, whereas in FIG. 14 most of the common pad connections are providedby the external probe mechanism 1401.

The fuses 1906 are included between grid lines 1905 and common padconnections 1907-1910 to provide for the case where a faulty die outputcannot be disabled by the test enable signal 609. For example, if thetester 401 sets the test enable signal 609 high to enable testing usingthe present invention, and the output pad of die 3 remains enabledoutputting a logic level, the fuse 1906 between gird line 1905 and theenabled output pad of die 3 will blow whenever the tester inputs anoppose logic level on grid line 1905. Without the fuse, the logic levelmaintained on the output pad of die 3 could prevent testing of the otherdie on wafer due to logic state contention on grid line 1905.Alternatively, a resistive element could be substituted for each fuse1906 to provide current limiting between a faulty die output pad andtester to enable testing of the other die. After testing and prior tothe die singulation step, the pad connecting grid lines, probe contacts,and fuses/resistive elements can be polished off the wafer 1901.

It should be noted that if wafers were processed to include the embeddedpad connection scheme shown on wafer 1901 of FIG. 19, tester 401 couldprobe multiple ones of the wafers 1901 at common probe contacts 1903 and1902 to enable simultaneous testing of multiple wafers 1901. Being ableto test multiple wafers simultaneously using one tester 401 would bringabout further reductions in test time and cost of manufacturing die. Forclarity, an example illustration of the above described multiple wafertest approach is depicted in FIG. 19A. In the example, tester 401 makescontact to probe contacts 1903 and 1902 of wafers 1901 via thepreviously described multiple wafer probe mechanism 1401 of FIG. 14. Theonly difference between FIGS. 14 and 19A is that in FIG. 14 multiple dieare tested whereas in FIG. 19A multiple wafers are tested.

While the present invention has been described thus far a being used tosimultaneously test multiple die on wafer and, as mentioned in regard toFIG. 19, even multiple wafers, it can also be used to simultaneouslytest multiple packaged ICs as well. FIG. 20 illustrates a test systemaccording to the present invention for simultaneously testing multiplepackaged ICs 1-N. The test system comprises a tester 401, a multiple ICprobe mechanism 2001, and identical packaged ICs 1-N to be tested. Inthis example, ICs 1-N each comprise a die 601, a package 2002 forholding die 601, bond wires 2003 for connecting the output pads 603 ofdie 601 to package output leads 2004, and bond wires 2005 for connectinginput pads 602 of die 601 to package input leads 2006.

The process of testing ICs 1-N in FIG. 20 is the same as that describedin the testing of die 1-N in FIG. 14. The only difference between thetwo tests is that the packaged die 601 of FIG. 20 are connected to theIC probe mechanism 2001 via bond wires 2005 and 2003 and input andoutput package leads 2006 and 2004. It is assumed in FIG. 20 that eachIC 1-N has package leads available for the test enable 609, scan control612, scan input 611, scan output 615, compare strobe 613, and failoutput 1302 signals. However, if not all the signals are available onpackage leads, they may be provided by sharing functional package leadsor by generating the signals internal to the die using test interfacessuch as the IEEE standard 1149.1 test access port interface.

What is claimed is:
 1. An integrated circuit comprising: A. input pads;B. output pads; C. core circuitry coupled between the input pads and theoutput pads; D. output circuitry coupled between the core circuitry andthe output pads, for each output pad the output circuitry including: i.a tri-state buffer having a core input lead connected to a core outputlead of the core circuitry, a data output lead connected to an outputpad and an enable input lead carrying an enable signal that can placethe data output of the tri-state buffer in a high impedance state; andii. comparator circuitry having a core input lead connected to the coreoutput lead and the core input lead of the tri-state buffer, an encodedresponse input lead connected to the output pad and the data output leadof the tri-state buffer, and an enable input lead connected to theenable input lead of the tri-state buffer.
 2. The integrated circuit ofclaim 1 in which the comparator circuitry includes a serial scan inputlead, a serial scan output lead, a serial scan control input lead and acompare strobe input lead.
 3. The integrated circuit of claim 1 in whichthe encoded response lead carries a trinary signal.
 4. The integratedcircuit of claim 1 in which the comparator circuitry includes a trinarygate having an input connected to the encoded response input lead, an ORgate having an input connected to the core input lead, and maskcircuitry connected to the outputs of the trinary gate and the OR gate.5. The integrated circuit of claim 4 in which the enable input lead iscoupled to the trinary gate, the OR gate and the mask circuitry.
 6. Theintegrated circuit of claim 1 in which the comparator circuitry includescompare gates, a pass/fail latch, and a scan cell, the compare gatesbeing connected to the core input lead and the encoded response lead,the pass/fail latch being coupled to the output of the compare gates tostore the result of a compare, and the scan cell being coupled to theoutput of the pass/fail latch and having an scan input lead, a scanoutput lead and a scan control lead.
 7. The integrated circuit of claim6 in which the pass/fail latch includes an AND gate coupled to thecompare gates and a flip-flop connected to the AND gate.
 8. Theintegrated circuit of claim 6 in which the scan cell includes amultiplexer connected to the output of the flip-flop, the scan inputlead and the scan control lead, and a flip-flop connected to the outputof the multiplexer, the scan output lead and the scan control lead.